Brownline of Prune Trees, a Disease Associated with Tomato Ringspot Virus Infection of Myrobalan and Peach Rootstocks

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Abstract
Surveys of declining prune orchards revealed a widespread incidence of prune brownline disease (PBL) associated with declining prune (P. domestica) trees on myrobalan plum (P. cerasifera) or peach (P. persica) rootstocks in several California [USA] prune-growing areas. A narrow strip of dark brown and necrotic cambial phloem tissues (brownline = BL) at the graft union of the scion and rootstock was the diagnostic symptom of the disease. Annual surveys of commercial prune orchards revealed natural spread of PBL from diseased to adjacent healthy trees. A virus (TomRSV[tomato ringspot virus]-P) serologically identical to the peach yellow bud (PYB) strain of TomRSV was isolated from and consistently detected by enzyme-linked immunosorbent assay test (ELISA) in myrobalan plum or peach rootstocks, but never in the prune scions of orchard trees naturally affected by PBL. TomRSV-PYB was readily graft-transmitted from naturally infected peach trees to myrobalan plum seedlings. The causal agent of PBL and TomRSV-P was graft-transmitted by root chips from myrobalan or peach rootstock of orchard-grown PBL-affected prune trees to healthy prunes (cultivar French) on myrobalan or peach rootstock only when the inoculum was applied to the rootstock of the indicators. Indicators inoculated in this manner developed typical BL at the union within 2 yr. Transmission of TomRSV-P or BL symptom induction was not achieved when root chips from either myrobalan or peach rootstock of PBL-affected prune trees were applied to the French prune scion of the indicators. Negative results also were obtained when buds from French scion of PBL-affected orchard trees served as inoculum and were applied either to the rootstock or French prune scion of the indicators. Apparently, TomRSV-P is present only in the myrobalan or peach rootstocks of naturally PBL-affected prune trees. The development of BL at the graft union of prune trees on myrobalan or peach rootstock seems to be due to a hypersensitive reaction of the prune scion to TomRSV-P.

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